Conventionally, an insulated gate field effect semiconductor device utilizing a thin film semiconductor (hereinafter referred to as a TFT) formed on an insulating substrate (especially a glass substrate) has been known. Such a TFT formed on an insulating substrate is utilized for devices such as liquid crystal displays and image sensors.
It is conventional that silicon oxide (SiO.sub.2) is utilized for a gate insulating film of the TFT mentioned above. It is desirable to utilize a crystalline silicon film as an active layer of the TFT so as to obtain excellent operating characteristics. Various methods can be used to form the crystalline silicon film, such as directly depositing a silicon film with a micro-crystalline structure by low pressure thermal CVD (chemical vapor deposition) and the like, or crystallizing an amorphous silicon film by a heating treatment or irradiating with laser light and the like.
In either case, it is impossible to obtain a single-crystalline structure at this stage. That is, a structure of a crystalline silicon film to be obtained is a polycrystalline structure, a microcrystalline structure, a mixed condition of a crystalline structure and an amorphous structure, or a structure including a crystalline structure.
In such a crystalline structure mentioned above, numerous dangling bonds of silicon exist, and therefore it is necessary that an active layer contain hydrogen to neutralize (i.e. terminate) the dangling bonds. That is, it is necessary to hydrogenize the active layer. On the other hand, it is imperative to prevent hydrogen from being included in the gate insulating film. That is, movable ions, such as hydrogen, existing in the gate insulating film during operation of a TFT cause hysteresis, or cause a variation in a threshold voltage value.
In the case of forming a conventional TFT on a glass substrate, the entire device tends to be charged with static electricity. Thus there is a problem that breakdown of the gate insulating film occurs due to the static electricity. That is, the charged static electricity generates a high voltage across the gate insulating film, and the gate insulating film cannot bear this voltage.
It is assumed that the above mentioned problem occurs because the energy band gap (Eg) of a silicon oxide (SiO.sub.2) film is as much as approximately 8 eV and the dielectric constant thereof is relatively small --approximately 3.8.
It can be considered to utilize a silicon nitride (Si.sub.3 N.sub.4) film with an Eg of approximately 5 eV and the dielectric constant of approximately 7 as a gate insulating film, instead of utilizing a silicon oxide film. However, if a silicon nitride film is utilized as a gate insulating film, hysteresis occurs to C-V (capacitance-voltage) characteristics, because Si clusters become charge trapping centers. Furthermore, there is a problem that the threshold voltage (.DELTA.V.sub.th) shifts about 10 V during a B-T (bias-temperature) treatment. That is, if silicon nitride is utilized as a gate insulating film, then charge trapping centers exist in the insulating film and therefore, this film is not preferable as an insulating film.
The invention disclosed herein is directed to:
(1) providing a gate insulating film in which breakdown caused by static electricity is reduced or eliminated, and
(2) providing a gate insulating film in which the existence of charge trapping centers is reduced or eliminated.
It is desirable to include hydrogen in an active layer. However, when hydrogen is included in the active layer, there is a problem that hydrogen is diffused from the active layer into the neighboring gate insulating film. This, however, is inconsistent with the requirement that hydrogen should not be included in the gate insulating film contacting the active layer. Therefore, the present invention further provides a gate insulating film to which ions in the active layer will not be diffused.
Also, when a material mainly comprising metal is utilized as a gate electrode, this metal material and the semiconductor component composing the gate insulating film undesirably form an alloy with each other. The invention disclosed herein is further directed at providing a structure wherein a metal component of the gate electrode is not diffused from the gate electrode into the gate insulating film.